Guidelines for Load Testing of Bridges (First Revision)
2014 Edition

IRC SP 51 specifies two primary load testing types for bridges: (1) Load Test for Rating and Posting, following IRC SP 37 guidelines, which assesses the bridge's strength and load capacity focusing mainly on flexural capacity over a short duration; and (2) Full Scale Load Testing of the superstructure, detailed within IRC SP 51 itself, designed to confirm the elastic performance of bridges under design live loads applied typically over 24 hours. This latter test serves for design validation and quality assurance, with emphasis on measuring flexural behavior accurately.

Deflections and strains during load testing are recorded at critical locations, such as mid-span and quarter-span for simply supported spans and beneath external ribs for box girders. Instruments like LVDTs with 0.01 mm precision, dial gauges, precision digital leveling devices, and total stations are employed to measure deflections. Strain gauges with microstrain accuracy capture strains, while inclinometers measure inclinations and thermometers track temperature changes. Measurements are taken before loading, during each loading stage, hourly during a 24-hour full load hold, throughout unloading, and after complete load removal to ensure comprehensive structural response monitoring.

The acceptance criteria specify the minimum percentage of deflection recovery within 24 hours after unloading, calculated using deflections before loading, under full load, and post unloading. The required recovery percentages are 75% for reinforced concrete bridges, 85% for prestressed concrete and steel bridges, and 75% for composite structures. These criteria ensure that the bridge exhibits primarily elastic behavior, with no excessive residual deflections compromising structural integrity.

Load application should proceed in increments of 50%, 75%, 90%, and then 100% of the test load, with each increment followed by a stabilization period of about one hour to allow deflections to settle. The unloading sequence reverses these steps: 100%, 90%, 75%, 50%, and finally 0%, also with stabilization pauses after each decrement. Both loading and unloading sequences are typically completed within 24 hours each. Testing must be halted immediately if any distress signs appear to ensure safety.

IRC SP 51 recommends using precise instruments such as Linear Variable Displacement Transducers (LVDTs) and dial gauges for deflection measurement, with least counts around 0.01 mm, and total stations or precision digital leveling instruments for positional accuracy. Strain gauges with microstrain resolution are used to track strain, inclinometers measure angular displacements, and digital or analogue thermometers monitor temperature changes with 0.5°C sensitivity. For dynamic response assessment, accelerometers combined with data acquisition systems record frequency, amplitude, and acceleration data. Proper placement and calibration of these instruments at critical locations are essential for accurate monitoring during load tests.